How can I tell if Blender is using all 64 GB of RAM?
You can tell how much memory Blender is currently using by looking in the info bar: 

You can use that to see if Blender's memory usage is hitting a plateau, but it seems to me your issue is your graphics card. More memory won't make your viewport faster, that's what a graphics card will do. More memory means Blender doesn't have to swap as much data onto the hard drive. That means more memory makes:


Getting cache for simulations faster (this won't necessarily make it run faster, it'll just make the data easier to grab). Apparently it also is necessary to compute high resolution fluids (thanks gandal3)
Viewing undo history faster (again, this is just accessing the data, not computing it).
Speed up loading large images (because it won't have to swap to the hard drive every time you adjust the image)
Video editor/tracker cache will also be stored on memory if it is available (which is of course faster than storing it on the hard drive) (thanks again to gandalf3).